The present invention relates to a floating apparatus for supporting an offshore platform. The apparatus of the invention includes a plurality of vertical columns attached to a submerged horizontal water entrapment plate on their lower end, and to a deck which supports minimum offshore facilities for the production of hydrocarbons offshore on their upper end. In another aspect, the present invention relates to methods for supporting minimum facilities required for the production of offshore hydrocarbon reservoirs from marginal fields.
With increasing exploration activities from offshore basins, such as the Gulf of Mexico, numerous discoveries of relatively small hydrocarbon accumulations have taken place. Many of these fields do not contain sufficiently large amount of oil or gas to justify the expenses of a stand-alone field development, such as a production platform and pipeline infrastructure. In many instances, however, these fields can be produced using subsea-tiebacks to existing infrastructure. These include a subsea wellhead and a flowline to an existing production platform for example.
Serious limitations are expected with longer subsea tie-back, such as plugging of the line due to a decrease in pressure and temperature along the flowline. Conventional remedial measures include injection of chemicals to prevent formation of hydrates. Such chemicals can be transported from the host platform to the subsea wellhead in an umbilical, and can be injected into the flowline at the wellhead. The umbilical can also be used to control the subsea wellhead. The cost of such umbilical is typically very large, and economics of a subsea tie-back is often threatened by the excessive umbilical cost for tie-back distances greater than 20 miles. An alternative development scenario consists of providing a minimum offshore platform near the wellhead with remote control from the host platform and injection of chemicals stored on the minimum offshore platform via a short umbilical connected to the subsea wellhead.
In some cases, where multiphase hydrocarbon flow is expected, the tie-back distance is further limited because of flow assurance issues. Current technological developments are aimed at providing subsea separation facilities to allow hydrocarbons to flow over a greater distance. Such subsea facilities may require additional surface facilities such as power generation and complex control capability.
Similarly, equipment such as subsea pumps may be required to assist flow assurance over the tie-back length. Such pump requires power which can be provided by a surface facility located above the pump.
Other technological solutions provided to the flow assurance problem for extended tie-back include electrically heated flowline, which may be heated either continuously or before start-up. The power required to heat the flowline may be produced by a generator located on minimum offshore facilities floating above the flowline.
Current technologies allow certain processing operations to be performed using much smaller equipment than traditional technologies. A minimum offshore platform could therefore be used to perform operations currently conducted on much larger platforms. This could further extend the distance over which hydrocarbon can be transported allowing them in cases to reach the shore directly for further processing.
A minimum offshore platform can also be used to perform basic maintenance workover on the wellhead. This saves the high cost of mobilization of a vessel suitable for typical workover operation.
Therefore, there is a need for minimum offshore platform in order to reduce the cost of development of marginal fields so as to make them profitable.
Different types of offshore platforms can be considered for production of hydrocarbons in deepwater. For example, Tension Leg Platforms (TLP's) are anchored to the seabed using vertical steel pipes, called tendons, which provide a large stiffness in the vertical direction. Mini-TLP's are smaller versions of TLP, but are typically not stable before they are connected to their tethers, and therefore the installation process is very complex, often requiring installation of Temporary Stability Modules as disclosed by Huang, U.S. Pat. No. 7,033,115, or installation of the deck and topsides offshore after the hull has been connected to its tethers. In very deep waters, much of the platform buoyancy is used to support the weight of the tendons, which reduces the payload-over-displacement ratio of these platforms.
Other existing floating platform concepts include deep draft caissons or spars, which are typically mono-column systems with a draft in excess of 400 ft. The draft is such that wave exciting forces are considerably reduced at the keel of the caisson. Because of the large size of the caisson, also referred to as “hard tank”, the amount of steel required to fabricate the platform is very large. A lighter version was proposed by Horton U.S. Pat. No. 5,558,467, wherein several horizontal water-entrapment plates were provided at a depth below which wave action does not contribute to heave motion. Topsides on these platforms must normally be installed offshore using heavy lift vessels.
Semi-submersible platforms, also referred to as column-stabilized platforms were initially designed to perform drilling activities offshore. These are composed of a plurality of vertical columns spaced a significant distance apart in order to provide stability. These platforms have also been used to support production facilities in deep- and ultradeep-water. Their displacement is in excess of 20,000 tons to achieve motion characteristics suitable for activities involved with the production of hydrocarbons offshore, such as support of risers, which are pipes carrying hydrocarbons or other fluids between the seabed and the process equipment located on the platform deck. These platforms can therefore carry a large payload, in excess of several thousands tons, but consequently their cost is high, and because of their large size, the required mooring system is also very large and costly.
Floating Production Storage and Offloading (FPSO) units constitute another type of floating platforms, which are ship-shaped, typically crude-oil carriers converted into production platforms. These are very large units. One of their advantages is the ability to store crude onboard the platform, and to offload to trading tankers.
All platforms described above must be very large to support production equipment in deep- or ultradeep-waters. Because wave loading increases with the size of the platform, the mooring or tendon system necessary to maintain these platforms on location is typically very large and costly. Thus, in spite of advancements in the art, there still exists a need for a low cost offshore platform to support relatively small payloads for the development of marginal offshore fields, which do not suffer from the disadvantages of the prior art apparatuses.
In order to develop smaller size or marginal fields, the weight of required process equipment is reduced, however existing platform concept cannot be easily scaled down because their motion performance tends to degrade considerably with smaller sizes. The platform of the present invention provides an alternative to existing platform concepts, which can be scaled down to the minimum required to support payload for small size hydrocarbon fields while retaining sufficient stability characteristics.
Column-stabilized platforms offer significant advantages over other platform types. Their motion characteristics are good compared with ship-shaped floaters. They can also be fully integrated at quayside, since their draft with topsides installed, is normally not very large, and they can be easily towed to their installation site.
The dynamics of offshore platforms can often be modeled using the relatively simple concept of harmonic oscillators, wherein the harmonic exciting force is provided by the oscillating wave force, the mass of the system is comprised of the platform mass as well as its added-mass, which is the amount of water entrained with the platform, the stiffness comes from hydrostatic effects, or changes in buoyancy with the motion of the platform, and damping comes from radiated waves or viscous effects. The Response Amplitude Operators (RAO) are mathematical expressions of the platform motion in its six-degrees of freedom—surge, sway, heave, pitch, roll and yaw—for incident waves of unit amplitude. The RAO go through a maximum when the platform natural periods, or resonant periods, coincide with the wave period. Designers aim at keeping the platform natural periods away from the most energetic wave frequency bands.
The size and spacing of columns of a semi-submersible platform is adjusted to provide the stability necessary to resist storm wind overturning moments, or destabilizing effects resulting from flooding of one of the columns. These dimensions are also adjusted to tune the platform dynamic response to the wave environment. In most offshore locations, wave periods vary between approximately 4 to 18 seconds. Typically the heave, pitch and roll natural periods are in excess of 20 seconds, so that dynamic amplification of the wave response only occurs for frequency bands where there is little wave energy. In certain areas, where very long period waves are found, it may be difficult to shift the heave natural period completely out of the wave energy range. Additional source of damping, such as disclosed by Leavitt (U.S. Pat. No. 3,397,545) can be provided to reduce the amplitude of the resonant response.
The heave natural period of a semi-submersible depends on the mass of the platform, its heave added-mass, and the column cross-sectional area. Similarly, pitch and roll natural periods depend on the platform inertia, its added-moment of inertia, which is the rotational equivalent of added-mass, and the platform metacentric height, GM. GM depends on the height of the center of mass, the center of buoyancy and the inertia of the waterplane area, which is related to the spacing of the columns and their size. The surge, sway and yaw properties of the platform can be adjusted with the mooring system.
As semi-submersible platform payload requirements are reduced and the platform get smaller, in order to keep the natural period in heave away from the wave spectrum, the column cross-sectional area may be reduced, which results in a reduction in the vertical stiffness, however this also results in a reduction of static stability. In order to compensate for the loss of static stability, the distance between columns can be increased, however the structural beams on the deck must be made stronger to support the increased span, resulting in significant weight increase of the platform deck. This undesirable “design spiral” is avoided by the present invention.